The polymerization of ethylene and the copolymerization of ethylene with other olefins is known to be carried out by gas phase, solution and/or suspension (slurry) polymerization processes. Advantages of the solution process include short reaction times, improved heat removal and monomer conversion for mass and energy control of polymerizations and single-phase reaction environments for controlling reaction dynamics. A most advantageous solution polymerization would be conducted at even higher reaction temperatures yet with a polymerization catalyst that yields sufficiently high molecular weight polymers with a high catalyst efficiency at these higher temperatures which lowers catalyst residues in the product and/or permits complete omission of the catalyst removal step.
In the suspension (slurry) polymerization of olefins, the advantages are low pressures, low temperatures and the ability to make very high molecular weight polymers. It is advantageous to carry out these reactions with sufficiently high polymerization efficiencies such that residues from the polymerization catalyst do not have to be removed from the resulting polymer.
There are many polymerization catalysts for suspension polymerization known in the art. Hagerty et al. in U.S. Pat. No. 4,562,169 disclose the preparation of a supported catalyst by treating a solid porous carrier having reactive OH groups such as silica in a liquid medium with an organomagnesium compound to react with the OH groups on the carrier; evaporating said liquid to precipitate magnesium onto the carrier and recovering a supported magnesium composition in the form of a dry, free-flowing powder; reacting the powder with a tetravalent titanium compound in a liquid medium. The catalyst is useful in the polymerization of olefins.
Nowlin in U.S. Pat. No. 4,593,009 and U.S. Pat. No. 4,672,096 discloses a catalyst for polymerizing olefins which catalyst is prepared by treating a carrier containing OH groups with an organomagnesium composition and contacting the thus-formed magnesium-containing carrier with a solution of at least one tetravalent vanadium compound or a solution containing both a vanadium compound and a titanium compound.
Gessel in U.S. Pat. No. 4,244,838 describes catalysts prepared from an organomagnesium compound, an organic hydroxyl-containing material and a transition metal halide. These solids produced by this reaction are isolated and washed prior to use in a polymerization.
Fuentes et al. in U.S. Pat. No. 4,544,647 disclose catalyst compositions prepared from an organomagnesium material, an organic hydroxyl-containing material, a reducing halide source and a transition metal compound.
Marchand et al. in U.S. Pat. No. 4,910,272 describe a process for polymerizing olefins in the presence of a catalyst prepared from an inorganic oxide, an organomagnesium material, an organic hydroxyl-containing material, a reducing halide source and a transition metal compound.
The catalyst efficiency of these catalysts is, in general, decreased with increased polymerization temperatures, specifically temperatures above 140.degree. C.
The catalysts known for solution polymerization comprise an organomagnesium component, an aluminum halide and/or an additional halide source and a transition metal compound. Lowery et al in U.S. Pat. No. 4,250,288 describes such compositions that are useful in the polymerization of .alpha.-olefins above 140.degree. C.
Sakurai et al. in U.S. Pat. No. 4,330,646 describes similar catalysts containing a titanium or a titanium and/or a vanadium compound as the transition metal component. These catalysts are useful at polymerization temperatures of at least 180.degree. C. The disadvantage of these catalysts is that the reactions that produce the catalyst solids are highly exothermic and difficult to control and reproduce. These catalyst compositions also contain a large excess of halide with respect to the transition metal component and yield polymers with a relatively high halide content. The composition as a whole is used directly in the polymerization of olefins.
It is well known in the art to optimize the properties of linear low density polyethylene (LLDPE) by variation in product molecular weight, molecular weight distribution (MWD) and density to match the required product application. Increasing the molecular weight, narrowing the MWD or lowering the density of LLDPE usually results in improved impact strength and puncture resistance properties. Molecular weight of the polymer prepared in Ziegler Natta catalyzed processes (as described by Professor Karl Ziegler in U.S. Pat. Nos. 3,113,115 and 3,257,332) is typically controlled in the process by the addition of varying amounts of telogens most commonly hydrogen. Similarly the density of the product is typically controlled by varying the comonomer concentration in the reaction medium.
In addition to optimizing product molecular weight and density for a given product application further improvement in resin performance can be obtained by narrowing the molecular weight distribution of a given melt index and density product. U.S. Pat. No. 4,612,300 describes a process for preparing LLDPE copolymers with narrow molecular weight distribution using a specific catalyst formulation, resulting in polymers for film applications with improved clarity and toughness.
Yet another property known to improve the clarity and toughness of alpha-olefin polymers is a small spherulite size as described for polypropylene (Kuhre et al, SPE Journal, October 1964, pps 1113-1119) and polyethylene (Narh et al, J. Mat. Sci, 15 (1980), pps 2001-2009). Similarly, U.S. Pat. No. 4,205,021 discloses copolymers with densities from 0.90 to 0.94 g/cm.sup.3 with exceedingly high weight average molecular weight but with the intrinsic viscosities of conventional ethylene copolymers and spherulite sizes of not more than six microns.
Linear low density polyethylene (LLDPE) produced with Ziegler catalysts have side groups introduced into the molecule from copolymerization with comonomers. In the case of 1-octene this side group would have six carbons atoms i.e. a hexyl chain. The distribution of these side groups or branches along and among all the polymer molecules is known as the polymer Short Chain Branching Distribution (SCBD) and the nature of this distribution has a strong impact on product properties and performance.
J. Plastic Film and Sheeting, Vol 4, July 1988, pps 214-226, "Short Chain Branching Distribution of ULDPE", K Dohrer L. G. Hazlitt, N. F. Whiteman), teaches that LLDPE copolymers prepared using a catalyst known to produce narrow molecular weight distribution LLDPE copolymers with a density greater than about 0.87 but less than 0.918 g/cm3 have unexpectedly improved (lower) amounts of n-hexane extractables than those from similar density products of other catalysts when having a narrow SCBD. Also in this study, a quantitative measurement of SCBD was introduced, the z.sub.1 /z.sub.0 or simply the z ratio. By this definition, polymers of similar densities should have similar values of z1 but polymers with a broader SCBD will have a lower value of z.sub.0. Therefore if z.sub.1 /z.sub.0 is low the SCBD is narrow and vice versa.
U.S. Pat. No. 4,438,238 discloses ethylene/alpha olefin copolymers with improved properties formed by mixing copolymers of high molecular weight and specified SCB (short chain branches/1000 carbons) with copolymers of lower molecular weight and specified SCB results in resins of 0.91 to 0.94 g/cm.sup.3 density and melt index of 0.02 to 50 g/10 min and melt flow ratio of 35 to 250 with excellent strength properties.
U.S. Pat. No. 4,918,038 discloses a process for the production of ethylene homopolymers or copolymers with a broad and/or bimodal molecular weight distribution using a mixed catalyst system. One advantage of this system is that the product can be made in a single reactor rather than using multistage reactors which raise questions of efficiency and cost.
U.S. Pat. No. 4,481,342 teaches a method of preparing an ethylene/alpha olefin copolymer of varying alpha olefin content, the incorporation of which is controlled by the porosity and pore radius of the magnesium chloride support.
U.S. Pat. No. 4,522,987 discloses a process using a chromium based catalyst system in which the incorporation of comonomer into the polymer chain occurs in a "super-random" fashion as described by the relative comonomer dispersity (RMD) as determined by N.M.R. The dispersity is controlled by the nature of the comonomer and varying its concentration in mole percent in the gas phase.
U.S. Pat. No. 3,645,992 discloses a continuous process for the preparation of homogeneous random partly crystalline copolymers of narrow MWD. The degree of homogeneity is controlled by varying the reactor temperature. Similarly homogeneity was decreased when R.sub.2 AlCl was used as cocatalyst rather than R.sub.1.5 AlCl.sub.1.5 or RAlCl.sub.2. Similarly increasing the ratio of cocatalyst to catalyst to greater than 9:1 for octene copolymers was required to yield homogeneous copolymers.
It would be desirable to have available catalyst compositions which exhibit significantly higher polymerization efficiencies based on the transition metal and the halide. It would also be desirable to have available catalyst compositions which exhibit these high efficiencies while being prepared in a manner which did not require the isolation and/or washing of the solid catalytic product. It would be further desirable to ease the process of preparation of the catalyst in order to increase reproducibility and quality of the catalyst.
It would also be desirable to have available such catalysts which would provide polymers having a high molecular weight and a relatively narrow molecular weight distribution and which exhibit more tolerance to hydrogen at polymerization temperatures of at least 180.degree. C. and even greater than 200.degree. C.
Also, it would be advantageous to have a solution process which, at a given melt index and density, results in a narrow molecular weight distribution product with small spherulite size, the SCBD of which, can be easily controlled to yield the desired combination of polymer properties for the specific product application.